Differential gene expression in the rat brain at the level of protein end-product was observed for a number of brain proteins, but the level of most brain proteins did not change with age. Differential gene expression also was observed for a number of brain mRNAs, but the majority did not change with age. The expression of the c-fos, c-myc and c-myb nuclear related oncogenes was examined in the rat brain. Sequences related to c- fos and c-myc but not to c-myb were detected. The expression of these sequences did not change with aging. In addition, the levels of B-actin mRNA did not change with age. Transcript levels of several proto-oncogenes were measured in brain and liver of rats by Northern blot hybridization analysis. An age- related increase in the level of c-myc mRNA in liver, but not brain, was observed. The levels of B-actin, c-sis, and c-src- related transcripts were age-invariant in both organs. The pp60c-src specific activity were measured in whole brain of Fischer-344 rats in an in vitro immune complex kinase assay. There was no significant difference in the pp60c-src specific kinase activity as a function of age. In situ hybridization methods were developed to measure the cellular levels of genes involved in cell signaling mechanisms. High levels of expression of c-myc and the glucocorticoid receptor (GR) genes were found in rat brain. Quantitative analysis indicated that the levels of GR transcripts were significantly different between contiguous but neuroanatomically-distinct regions of the hippocampus.